This invention relates to nonaqueous galvanic cells, and more particularly to a construction for such cells that substantially reduces or eliminates self-discharge of the cell with time.
Galvanic cells are often constructed so that the cell container is used as one of the electrode terminals of the cell. In consequence, it is necessary to dispose an electrical insulator between the cell container and the other cell electrode terminal to prevent the cell from shorting.
In addition, galvanic cells typically are sealed to prevent leakage and consequent loss of electrolyte, and thus in the cell construction described above the electrical insulator should be securely bonded in a leakproof manner to both the cell container and the other cell electrode terminal. However, a consequence of such sealing is that certain operating conditions can cause the internal pressure of the cells to markedly increase. In cells utilizing a highly reactive anode material such as lithium, external sources such as fire or internal sources such as heat generated during charging can cause the anode to melt and vigorously react with the cathode and/or electrolyte, thereby resulting in a sharp increase in internal cell pressure. In the case of other galvanic cells, such as alkaline zinc cells, carbon zinc cells, etc., large quantities of gas are generated under certain conditions of use. Thus, if any of the foregoing cells were permanently sealed, the build up of internal pressure within the cell could cause the cell container to leak, bulge or even rupture, with the attendant possibility of property and/or bodily damage.
It is therefore necessary to provide a vent for galvanic cells which remains sealed during normal operating conditions, but which opens when the pressure within the cell substantially increases. To meet these objectives, cells have been made with a vent release mechanism. Referring to the situation where an electrical insulator is disposed between the cell container and a cell electrode terminal, the insulator can act both to isolate the cell container from a cell electrode terminal and as a vent release mechanism. Specifically, the insulator can be made of glass or ceramic material that is sufficiently thin so as to be frangible. The insulator is then disposed within and secured to a vent orifice that is usually located in the cell cover, so as to hermetically seal the vent orifice, and the cell electrode terminal passes through the central region of the insulator. When the pressure within the cell exceeds a predetermined limit, the frangible member fractures to release the excess pressure. In one type of cell, referred to as a flat cell, a short, cylindrical container holds a wafer-like anode comprising an active anode material, such as lithium, disposed over and separated from a wafer like cathode comprising an active cathode material, such as manganese dioxide. A ferrous metal, such as stainless steel, is commonly used for the container. This is because stainless steel is generally corrosion resistant, is easily formed or machined into an appropriate container shape and is electrically conductive so that the container itdischarge can form one terminal of the cell.
A container cover disposed over and separated from the anode is hermetically sealed to the cell container. The cathode is disposed to rest on the bottom of the cell container, thereby making the container the positive electrode terminal of the cell. In contrast, the anode is electrically isolated from the container.
So that electrical contact can be established with the anode, a disk-shaped current collector plate is disposed over and placed in physical (and thus electrical) contact with the anode, and a collector insulator is placed between the current collector plate and the cover to maintain the electrical isolation of the anode from the container. A cylindrical pin typically made of a ferrous material, such as stainless steel, is placed in electrical contact with the current collector plate and disposed to protrude through an orifice in the cell cover to form the negative electrode terminal of the cell. An annular seal typically made of glass is disposed within the orifice between the pin and the cell cover to hermetically seal the cell. This seal will fracture when pressure within the cell substantially increases, thereby relieving the pressure.
Corrosion problems in the foregoing cell construction have arisen in connection with the glass seal. Specifically, in the case of alkali metal anodes, especially lithium, it has been found that during storage a conductive corrosive deposit grows from the negative electrode terminal pin across and into the seal undersurface toward the cell container, which is the positive electrode terminal of the cell. This deposit grows until the glass seal is bridged and the cell is shorted, thereby causing the cell to self-discharge. Moreover, during the course of its growth the deposit corrodes the glass seal, which gives rise to the possibility of cell leakage.
While the exact nature and cause of the conductive corrosive deposit are not known, it is believed to be a lithium-modified ferrous compound caused by a complex reaction that is at least a function of the cell potential and the material compositions of the anode, those portions of the cell structure that are in electrical contact with the cathode material, and the glass seal.
Efforts to prevent premature failure of the cell and thereby prolong the shelf life of lithium cells have for the most part concentrated on the seal composition and/or effective coatings for the seal.
For example, Sandia Report #83-2314 of September, 1984, "Glass Corrosion in Liquid Lithium", suggests that certain glass compositions are better able to withstand corrosion by liquid lithium than others. In U.S. Pat. No. 4,168,351, corrosion of a seal is retarded by coating the entire glass surface exposed to the interior of the cell with a protective material such as a metal oxide, polyolefin or fluorocarbon polymer. In U.S. Pat. No. 4,233,372, an inert polymeric coating is applied over the glass surface exposed to the cell environment to reduce chemical attack on the glass, and in European Pat. No. 35,074, the exposed glass surface is protected by a silicone layer. A still further solution to the problem of glass corrosion is proposed by U.S. Pat. No. 4,308,323, wherein the resistance of the glass to chemical attack is improved by a graded seal composed of one glass composition bonded to the terminal pin and another glass composition bonded to the wall of the container.
An alternative approach to solving the corrosion problem is to be found in U.S. Pat. No. 4,609,598, issued to Gary Tucholski and Earl Chaney, Jr., the same inventors as herein, and assigned to the assignee of this invention. In that patent, all metal components of the cell electronically connected to the cathode are made of a non-ferrous metal, such as molybdenum. This construction decreases the deposition of conductive corrosive material on the glass seal, and thus the resulting seal corrosion.